1. Field
Example embodiments relate to a method and apparatus of correcting error data used in a non-volatile memory device, for example, to a method and apparatus of correcting error data caused by charge loss.
2. Description of the Related Art
A non-volatile memory device may retain stored data even when not powered. Examples of a non-volatile memory device may include flash memory, read-only memory, most types of magnetic computer storage (e.g., hard-disks, floppy disk drives, and magnetic tapes), and optical disc drives. For instance, flash memory, such as NAND, may be a type of flash memory capable of storing large amounts of data in a small area.
FIG. 1A illustrates a conventional memory cell of a non-volatile memory device. FIG. 1B illustrates a graph indicating the threshold voltage of a conventional non-volatile memory device. FIG. 1C illustrates a conventional memory cell of a non-volatile memory device when electrons are injected in the floating gate. Memory cells of a conventional non-volatile memory device may include a cell transistor having a control gate CG, a floating gate FG, a source, a substrate (bulk) and a drain.
Threshold voltage of a memory cell transistor within a non-volatile memory device defines the stored logic of the memory cell. For example, when a memory cell transistor is in its initial state (also called an erased state), the threshold voltage Vth may be relatively low. In this state, the memory cell transistor may be designated to have a logic value “1.” On the other hand, when the memory cell transistor is in its programmed state, the threshold voltage Vth may be relatively high. This high threshold voltage state may be designated to have a logic value “0.” Referring to FIG. 1B, the threshold voltage in the program state may be larger than 0, and the threshold voltage in the erase state may be smaller than 0.
The cell transistor of a memory cell may be programmed or erased by an F-N tunneling mechanism. F-N tunneling, or field emission, is the process whereby electrons tunnel through a barrier in the presence of a high electric field.
In order to change a memory cell transistor from its erased state to its programmed state through F-N tunneling, a voltage higher than the source voltage can be applied to the control gate CG. Referring to FIG. 1C, a relatively large positive potential difference is created between the control gate CG and the substrate (bulk), and excited electrons within the channel on the surface of the bulk are forced through and trapped in the floating gate FG. In other words, electrons may be injected into the floating gate FG. These negatively charged electrons act as a barrier between the control gate and the channel on the bulk, thereby increasing the threshold voltage of the memory cell transistor.
The memory cell can be brought back to its erased state by applying a voltage higher than a source voltage to the bulk. Thereby, forming a large negative potential difference between the control gate CG and the bulk. F-N tunneling draws the trapped electrons back, thus removing the electron barrier and decreasing the threshold voltage Vth.
FIG. 2 illustrates a programming operation of a conventional memory cell of a non-volatile memory device due to charge loss.
As time elapses, electrons injected into the non-volatile memory cell may be lost. Referring to FIG. 2, the electrons trapped in the floating gate FG are lost as time elapses. The result is fewer electrons within the floating gate FG. When electrons of the floating gate are lost, the threshold voltage of the non-volatile memory cell decreases. When electrons are lost, the threshold voltage may decrease As a result, the programmed state may be confused with the erase state thereby causing memory cell malfunctions.